Allow Custom Slug for Subject & Node Creation

[trtajim]

Summary

Currently, slugs are generated automatically from the title. When the title is in Bangla, the generated slug is often unreadable or undesirable.

Proposed Change

• Add slug to the controller validation for both Subject and Node creation.
• Accept the slug from the frontend instead of always generating it automatically.
• Use the provided slug when creating the record.

Acceptance Criteria

• Subject creation accepts a custom slug.
• Node creation accepts a custom slug.
• Slugs are validated for uniqueness and format.
• Automatic slug generation is no longer required when a slug is provided.